They should of course find a solid replacement for Mbappe. I assume they have started working on it already. There are quite talented young players in the squad actually but we couldn't see a very impressive performance from them so far. I have bigger expectations from players such as Kolo Muani, Gonçalo Ramos and so on. I should exclude Vitinha from them though. Because I'm really impressed with him. He is one of the best contributors in the squad now.
Sure they will start looking for Mbappes replacement so that they will start breeding the player if he would be able to even play better than Mbappe. So that his absence won't affect there performance that much. And I hope is going to Madrid his legit then his talent will be well utilized there and there might be a good chance of winning the Champions League. Kolo Muani, Gonçalo Ramos are excellent players but also need to work on there selfes the more.